Rotary screw pumping and turbulent flow integrated heat dissipation device for satellite-borne high-power equipment

Abstract

The invention discloses a rotary screw pumping and turbulent flow integrated heat dissipation device for satellite-borne high-power equipment, which can simultaneously realize pumping and turbulent flow of cooling liquid through rotation of a screw so as to achieve a better heat dissipation effect, and comprises a cooling pipe, a rotary screw and a motor, the cooling pipe can be used for introducing a cooling working medium and is provided with an outer heat-absorbing surface and an inner heat-absorbing surface which are parallel to each other; wherein the outer heat absorption surface is attached to the heat dissipation surface of the satellite-borne high-power equipment, and the inner heat absorption surface is located on the inner wall of the cooling pipe and is adjacent to the outer heat absorption surface; the rotating screw rod comprises a central rod and a spiral blade; one end of the center rod extends out of the cooling pipe and is connected with a motor, and the height of the motor can be adjusted. The lead angle of the spiral blade is alpha, the curvature radius of the spiral blade is R, the minimum gap from the outer wall face of the spiral blade to the inner heat absorption face is delta, and the rotating speed of the motor is v, so that efficient removal with the surface heat flux density larger than or equal to 50 W / cm < 2 > of the satellite-borne high-power equipment can be achieved through cooperative control over alpha, R, delta and v, and the mass specific power consumption does not exceed 0.1 W / g during heat dissipation.

Description

Technical Field

[0001] This invention relates to the field of spacecraft thermal control technology, and in particular to an integrated heat dissipation device for high-power spaceborne equipment using a rotary screw pump with turbulence. Background Technology

[0002] Currently, when low-Earth orbit satellites are in orbit, the multi-core processors of the onboard high-power-density chips will face the following technical bottlenecks due to the surge in transient computing power:

[0003] 1. Localized hot spots, which in turn lead to excessive temperature gradients.

[0004] 2. The thermal boundary layer inside the cooling pipe thickens, which in turn leads to a decrease in heat transfer efficiency.

[0005] 3. Traditional mechanical pump drive systems have excessively high power consumption per unit mass, such as exceeding 2.5W / g.

[0006] With the use of new-generation spaceborne high-power-density chips, there will be higher requirements for heat flux density, temperature uniformity, and mass-to-power ratio. For example, the following three conditions need to be met simultaneously:

[0007] A. High heat flux density: The heat flux density during chip heat dissipation needs to exceed 50W / cm². 2

[0008] B. High temperature uniformity: It is required that the heat dissipation of the chip is uniform and the temperature difference does not exceed 105K, so that the maximum temperature in the chip does not exceed the fatigue limit of the solder joint material.

[0009] C. Low power consumption per unit mass: The power consumption per unit mass during chip heat dissipation does not exceed 0.1W / g.

[0010] In existing technologies, spacecraft chip cooling typically utilizes a mechanically driven ribbed cooling pipe system (also known as a ribbed structure) for heat dissipation. Specifically, this involves a stainless steel gear pump and ball bearing mechanical drive system, with torque transmitted directly to the impeller via a coupling. Rectangular ribs are periodically arranged within a straight cooling pipe, leveraging flow separation to enhance turbulent mixing. The heat dissipation medium is an ethylene glycol-water mixture, relying on an external heat exchanger for heat sink cooling. The overall system structure uses an aluminum alloy cooling tube sheet, connected to the pump body via brazing to form a forced convection cooling system.

[0011] The aforementioned rib structure has the following key drawbacks in spacecraft thermal control applications:

[0012] (1) Insufficient heat flux density load-bearing capacity, specifically manifested as:

[0013] A. Effective heat transfer area loss: Flow separation in the wake region of the finned section reduces the actual effective heat transfer area by 38-45%.

[0014] B. Insufficient thermal boundary layer suppression: when heat flux density > 80 W / cm² 2 At that time, the thermal boundary layer thickness δ≥0.25mm

[0015] C. Heat flux density bottleneck: The system's maximum heat dissipation capacity is limited to 95W / cm². 2 It cannot meet the requirement of ≥100W / cm² for next-generation spaceborne high-density equipment. 2 Heat dissipation requirements

[0016] (2) Severe temperature distribution imbalance: The uneven flow distribution caused by the rib structure leads to an inter-core temperature gradient ΔT>45K, exceeding the fatigue limit of the solder joint material. In addition, the dynamic response is slow, and the temperature stabilization time under step heat load is 90s.

[0017] (3) The mass-to-power ratio is too high, specifically manifested as:

[0018] A. Excessive flow resistance: The protruding rib structure causes a pressure drop ΔP ≥ 220 Pa.

[0019] B. Severe mechanical losses: Frictional power consumption of ball bearings accounts for 32-37% of the total system power consumption.

[0020] C. Exceeding the standard for mass-to-power ratio: The overall mass-to-power ratio of the system reaches 2.8-3.2W / g, far exceeding the requirement of 0.5W / g for the new generation of high-power spaceborne equipment.

[0021] In addition, traditional solutions rely on high flow rate conditions (working fluid velocity v≥3.5m / s) to enhance heat transfer, but the pressure drop shows superlinear growth, which cannot meet the spacecraft's requirements for high heat flux and low power consumption. Summary of the Invention

[0022] The technical problem to be solved by the present invention is to address the shortcomings of the prior art by providing a rotary screw pump integrated heat dissipation device for high-power spaceborne equipment. This rotary screw pump integrated heat dissipation device can achieve a surface heat flux density of ≥50W / cm² for high-power density spaceborne chips. 2 The efficient removal of heat and the mass-to-power ratio during heat dissipation do not exceed 0.1W / g.

[0023] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0024] A rotary screw pump turbulence integrated heat dissipation device for high-power spaceborne equipment includes a cooling pipe, a swirling turbulence screw, and a motor.

[0025] The cooling tube can be used to introduce a cooling medium and has an outer heat-absorbing surface and an inner heat-absorbing surface that are parallel to each other. The outer heat-absorbing surface is attached to the heat dissipation surface of the spaceborne high-power-density chip, and the inner heat-absorbing surface is located on the inner wall of the cooling tube and adjacent to the outer heat-absorbing surface.

[0026] The swirl disturbance screw is axially inserted into the center of the cooling tube. The swirl disturbance screw includes a central rod and spiral blades spirally arranged on the outer periphery of the central rod.

[0027] One end of the central rod extends out of the cooling tube and is connected to the motor. The height of the motor can be adjusted, and it drives the central rod to rotate.

[0028] Let the lead angle of the spiral blade be α, the curvature radius of the spiral blade be R, the minimum clearance from the outer wall surface of the spiral blade to the inner heat-absorbing surface be δ, and the rotational speed of the motor be v. Then, through the coordinated control of α, R, δ, and v, the heat flux density during the heat dissipation of the spaceborne high-power density chip can be made not lower than 50 W / cm 2 , and the specific mass power consumption does not exceed 0.1 W / g.

[0029] The lead angle α of the spiral blade is positively correlated with the rotational speed v of the motor. Among them, 5000 rpm ≤ v ≤ 7000 rpm, 35° ≤ α ≤ 50°. The optimal value of the rotational speed v of the motor can make the axial pumping flow rate Q of the swirl disturbance screw not lower than 15 mL / s, and can also induce secondary swirl through the curvature of the spiral blade.

[0030] v = 6000 ± 500 rpm, α = 45 ± 5°.

[0031] R = 0.4 - 0.7r; where r is the radius of the central rod.

[0032] When α and R remain unchanged, by increasing v and decreasing δ, the heat flux density during the heat dissipation of the spaceborne high-power density chip can be made not lower than 50 W / cm 2 , and the specific mass power consumption does not exceed 0.1 W / g.

[0033] δ = 0.2 - 0.3r; where r is the radius of the central rod.

[0034] Let the thickness of the spiral blade be t, then t = 0.3 ± 0.05 mm.

[0035] The cross-section of the cooling tube is in the shape of a "hui" character, and the roughness Ra of the inner wall surface of the cooling tube ≤ 0.8 μm.

[0036] The material of the swirl disturbance screw is SiC-TiC ceramic matrix composite, and the material of the cooling tube is high-purity AlN ceramic.

[0037] Through the coordinated control of α, R, δ, v, and t, the heat flux density during the heat dissipation of the spaceborne high-power density chip can be made not lower than 50 W / cm 2 , and the specific mass power consumption does not exceed 0.1 W / g.

[0038] This invention offers the following advantages: Through an innovative design integrating a swirling disturbance screw within a straight channel, it achieves a unified pumping and turbulence function. When the screw rotates, its helical blades simultaneously generate axial pumping force (driving directional flow of the working fluid) and tangential shear disturbance (inducing transverse secondary flow), forming a Dean-like vortex junction within the straight channel. This design overcomes the limitations of traditional straight-channel laminar flow at the bottom layer. Under Re=3000 conditions, the near-wall velocity gradient is increased, reducing the thermal boundary layer thickness, significantly increasing the Nusselt number, and achieving a heat flux density exceeding 50 W / cm². 2 Meanwhile, the continuous shearing action of the swirling disturbance screw suppresses flow separation, reducing the system's specific power consumption to 0.1 W / g, achieving synergistic optimization of high heat flux removal and low power consumption. Attached Figure Description

[0039] Figure 1 This image shows a three-dimensional line drawing of the integrated cooling device for rotary screw pumping turbulence in spaceborne high-power equipment according to the present invention.

[0040] Figure 2 The image shows a three-dimensional simulation of the integrated cooling device for turbulence-induced heat dissipation using a rotary screw pump for high-power spaceborne equipment according to the present invention.

[0041] Figure 3 Showing Figure 1 A longitudinal section along the axial direction.

[0042] Figure 4 Showing Figure 1 A longitudinal section perpendicular to the axial direction.

[0043] Figure 5 A partially enlarged schematic diagram of the swirling disturbance screw is shown.

[0044] Among them are:

[0045] 10. Cooling pipe; 11. External heat-absorbing surface; 12. Internal heat-absorbing surface;

[0046] 20. Swirl-induced disturbance screw; 21. Central rod; 22. Helical blade;

[0047] 30. Electric motor; 31. Pump base;

[0048] 40. Spaceborne high power density chips. Detailed Implementation

[0049] The present invention will now be described in further detail with reference to the accompanying drawings and specific preferred embodiments.

[0050] In the description of the present invention, it should be understood that the orientation or positional relationship indicated by terms such as "left side", "right side", "upper part", "lower part", etc. is based on the orientation or positional relationship shown in the drawings. It is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. "First", "second", etc. do not represent the importance of components, so it cannot be understood as a limitation to the present invention. The specific dimensions adopted in this embodiment are only for illustrating the technical solution by way of example and do not limit the protection scope of the present invention.

[0051] As Figure 1 shown, a rotating screw pumping and turbulence integrated heat dissipation device for spaceborne high-power equipment includes a cooling pipe 10, a swirling and disturbing screw 20, and a motor 30.

[0052] The above-mentioned cooling pipe can be used to introduce a cooling working medium and has mutually parallel outer heat absorption surfaces 11 and inner heat absorption surfaces 12; among them, the outer heat absorption surface is attached to the heat dissipation surface of the spaceborne high-power density chip 40, and the inner heat absorption surface is located on the inner wall of the cooling pipe and is adjacent to the outer heat absorption surface.

[0053] The encapsulation of the above-mentioned spaceborne high-power density chip preferably has the following two methods:

[0054] (1) Eutectic welding: Au80Sn20 solder, welding temperature 320°C ± 5°C, thermal resistance R th = 0.08 K·cm 2 / W.

[0055] (2) Glass sealing: Borosilicate glass (CTE = 4.3×10 -6 / K), helium leak detection rate: <5×10 -9 Pa·m 3 / s.

[0056] Furthermore, the cross-section of the cooling pipe preferably has a "return" shape, and the inner cavity cross-section is preferably square. As an alternative, the cooling pipe can also be rectangular. When it is rectangular, the narrow bottom side of the outer heat absorption surface is the outer width W of the cooling pipe, and the outer height of the cooling pipe perpendicular to the heat dissipation surface is H. Then preferably W = 300 ± 10 μm, height H = 500 ± 15 μm, width-to-height ratio AR = W / H = 0.6, and the optimal hydraulic diameter D h = 375 μm.

[0057] Furthermore, the roughness of the inner wall surface of the cooling pipe is preferably Ra ≤ 0.8 μm. When electro-polished to Ra < 0.05 μm, through simulation, it shows that the flow resistance is reduced by 23%.

[0058] Furthermore, the material of the cooling pipe is preferably high-purity AlN ceramic, with the following physical properties: thermal conductivity κ = 180 W / m·K, linear thermal expansion coefficient CTE = 4.6 ppm / K, purity > 99.9%, and three-point bending strength σ. b =620MPa.

[0059] The aforementioned cooling pipe is preferably manufactured using SLM laser 3D printing. The specific manufacturing method includes the following steps.

[0060] Step A1: Prepare the substrate: high-purity AlN ceramic.

[0061] Step A2: Set SLM parameters: laser power P = 150W, scanning speed v = 800mm / s, layer thickness h = 20μm, porosity <0.05% (metallographic inspection).

[0062] Step A3, Nanopolishing: The inner wall surface of the 3D-printed cooling tube is nanopolished using a diamond suspension; the preferred diamond particle size is 50 nm. After nanopolishing, the surface roughness Ra of the cooling tube inner wall is reduced from 0.8 μm to 0.05 μm using a white light interferometer.

[0063] The aforementioned swirling disturbance screw is inserted axially into the center of the cooling pipe. The swirling disturbance screw includes a central rod 21 and helical blades 22 arranged spirally on the outer periphery of the central rod.

[0064] One end of the aforementioned central rod extends from the cooling pipe and is connected to the motor 30. The height of the motor is adjustable, and it drives the central rod to rotate. The method of adjusting the height of the motor is existing technology. In this embodiment, the tail end of the motor is preferably slidably mounted in the pump base 31 via a connecting block and a slider.

[0065] The aforementioned swirling disturbance screw is a symmetrical helical screw structure, and its material is preferably SiC-TiC ceramic matrix composite material. The corresponding physical parameters are preferably: thermal conductivity k = 180 W / m·K, bending strength σ b =620MPa, coefficient of thermal expansion CTE =4.2×10 -6 / K, achieving thermal-mechanical matching; verified through ANSYS static simulation: maximum equivalent stress σ vm =185MPa (safety factor S=3.35), thermal deformation ΔL=7.2μm.

[0066] The surface roughness of the aforementioned swirling disturbance screw is preferably Ra≤0.8μm. The aforementioned swirling disturbance screw is preferably formed by CIM: the feed solid content is 92vol%, the degreasing slope is 0.5℃ / min (to prevent cracking), and after forming, it is ground with diamond resin combined with a grinding wheel, with a roundness error <0.5μm.

[0067] like Figure 4 and Figure 5 As shown, let the lead angle of the helical blade be α, the radius of curvature of the helical blade be R, the minimum gap between the outer wall of the helical blade and the inner heat-absorbing surface be δ, and the motor speed be v. Then, by controlling α, R, δ, and v in a coordinated manner, the heat flux density of the spaceborne high-power-density chip during heat dissipation can be made no less than 100 W / cm². 2 The mass-to-power consumption ratio does not exceed 1.0W / g.

[0068] Furthermore, assuming the thickness of the helical blade is t, in this embodiment, t is preferably 0.3 ± 0.05 mm; through the coordinated control of α, R, δ, v, and t, the heat flux density during heat dissipation of the spaceborne high-power-density chip can be made not less than 100 W / cm². 2 The mass-to-power consumption ratio does not exceed 0.5W / g.

[0069] The lead angle α of the aforementioned helical blades is positively correlated with the motor speed v, wherein 5000rpm≤v≤7000rpm and 35°≤α≤50°. The optimal value of the motor speed v ensures that the axial pumping flow rate Q of the swirling disturbance screw is not less than 8mL / s, and also induces secondary swirling flow through the curvature of the helical blades. In this embodiment, v = 6000±500rpm and α = 45±5° are preferred.

[0070] Further, R = 0.4~0.7r, δ = 0.2~0.3r, where r is the radius of the central rod; in this embodiment, R = 0.35±0.02mm, δ = 0.1~0.2mm is preferred.

[0071] With α and R remaining constant, by increasing v and decreasing δ, the heat flux density of the spaceborne high-power-density chip during heat dissipation can be maintained at no less than 50 W / cm². 2 The mass-to-power ratio does not exceed 0.1W / g.

[0072] Working principle of the invention

[0073] (1) Structural features

[0074] The swirling disturbance screw adopts a constant lead angle design (α=45±5°). Through precision machining, the blade thickness t=0.3mm and the gap with the cooling pipe δ=0.1mm are ensured. The optimized lead angle of 45° ensures that the axial pumping flow rate Q≥15mL / s and induces secondary swirling through the curvature of the spiral wall.

[0075] (2) Fluid dynamics mechanism:

[0076] When the swirling disturbance screw rotates at a speed of n = 6000 ± 500 rpm, it generates axial pumping force and circumferential shear stress (τ). w=17.38Pa), inducing the formation of Taylor-like vortices. Fluent transient simulation verified that at Re=3338, the Nu number reached 12.32, which is a significant improvement over the traditional protruding rib structure.

[0077] A. Heat-fluid synergy:

[0078] A symmetrical helical rod structure (radius of curvature R = 0.35 ± 0.02 mm) was adopted, and the results were verified through Fluent conjugate heat transfer simulation at a heat flux density of 100 W / cm². 2 Under operating conditions, the maximum wall temperature Tmax = 402.92 K (ΔT = 21 K safety margin), and the temperature difference σ T =105, effectively suppressing boundary layer thickening.

[0079] B. Fluid-force synergy:

[0080] By optimizing the topology matching between the lead angle (α = 45° ± 5°) and the cooling pipe gap (δ = 0.1 mm), a deep coupling between fluid dynamics and structural mechanics is achieved, specifically manifested as follows:

[0081] ① A 45° lead angle makes the axial velocity component v z With tangential component v θ The ratio is 1.3:1.15, which forms a stable helical secondary flow and suppresses flow separation.

[0082] C. Flow field control mechanism of the screw rod:

[0083] (1) The synergistic effect of swirling disturbance screw-induced axial pumping and circumferential secondary vortex.

[0084] (2) Eddy field reconstruction mechanism, breaking through the traditional laminar bottom layer limitation.

[0085] D. Cooling pipe geometry optimization: The synergistic design of aspect ratio and screw clearance increases the surface Nusselt number to 12.32.

[0086] E. Verification of Thermal-Fluid-Structure Coupling Performance

[0087] (1) Dynamic response characteristics: The temperature stabilization time under step heat load is 22s, while the traditional solution is 90s.

[0088] (2) Extreme operating condition tolerance: performance degradation <5% under 25g RMS vibration.

[0089] This invention utilizes a three-dimensional flow field manipulation technique with a rotating helical rod within a straight channel to establish a transient conjugate heat transfer model in Fluent that includes a rotational domain. Simulation results show that:

[0090] I. Capable of exceeding 50W / cm 2 high heat flux density

[0091] The thermal boundary layer suppresses the secondary vortex induced by the circumferential velocity component of the screw, reducing the thermal boundary layer thickness to 0.08 mm and increasing the Nusselt number to 12.32.

[0092] Flow separation control: Streamline tracing shows that the separation zone area accounts for less than 10%.

[0093] II. Significantly improved temperature uniformity

[0094] Pressure self-balancing: The axial momentum transmission of the screw rod makes the inlet pressure difference ΔP < 15Pa;

[0095] Temperature homogenization; 50W / cm 2 Under the condition of heat flux, the temperature difference is controlled at around 100K.

[0096] III. Mass-to-power consumption is less than 0.1W / g

[0097] The existing solution has high flow resistance, which causes the pump power to account for 68-73% of the total power consumption, and the system power consumption exceeds the standard by 47%.

[0098] This invention suppresses flow separation and optimizes pressure drop characteristics by using the curvature of the spiral flow channel. Under the same heat flux density, the flow rate requirement is reduced, and combined with the magnetic levitation drive efficiency η = 95%, the system's specific power consumption is reduced. The optimal spiral rod configuration (lead angle α = 45° ± 5°, clearance δ = 0.1–0.2 mm) is obtained through Fluent parameter optimization.

[0099] Flow resistance characteristic reconstruction: at v = 3 m / s, ΔP = 82 kPa, which is greatly reduced compared with the existing technology;

[0100] Power consumption compliance verification: Combined with magnetic levitation drive (efficiency η = 95%), the system mass-to-power consumption is reduced to 0.35W / g, which meets the spacecraft requirement of ≤0.5W / g.

[0101] In addition, the structural reliability under 15g RMS (10-2000Hz) random vibration conditions is improved, while the system mass-to-power consumption is reduced to <1.8W / g, which is significantly lower than that of traditional solutions.

[0102] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various equivalent transformations can be made to the technical solutions of the present invention, and these equivalent transformations all fall within the protection scope of the present invention.

Claims

1. A rotary screw pump integrated cooling device for high-power spaceborne equipment, characterized in that: It includes a cooling pipe, a rotating screw and a motor; The cooling pipe can be used to introduce a cooling working medium and has mutually parallel outer heat-absorbing surfaces and inner heat-absorbing surfaces; wherein, the outer heat-absorbing surfaces are attached to the heat-dissipating surfaces of the spaceborne high-power equipment, and the inner heat-absorbing surfaces are located on the inner wall of the cooling channel and are adjacent to the outer heat-absorbing surfaces; The rotating screw is axially inserted into the center of the cooling pipe. The screw includes a central rod and spiral blades spirally arranged on the outer periphery of the central rod; one end of the central rod extends out of the cooling pipe and is connected to the motor. The height of the motor can be adjusted, and the motor drives the central rod to rotate; Let the lead angle of the helical blade be α, the radius of curvature of the helical blade be R, the minimum clearance between the outer wall of the helical blade and the inner heat-absorbing surface be δ, and the motor speed be v. Then, by coordinating the control of α, R, δ, and v, the heat flux density of the high-power-density chip on space can be made no less than 50 W / cm². 2 The mass-to-power consumption ratio does not exceed 1.0W / g.

2. The integrated cooling device for rotary screw pumping and turbulence dissipation used in spaceborne high-power equipment according to claim 1, characterized in that: The lead angle α of the spiral blades is positively correlated with the motor speed v. Among them, 5000 rpm ≤ v ≤ 7000 rpm, 35° ≤ α ≤ 50°. The optimal value of the motor speed v can make the axial pumping flow rate Q of the swirling disturbance screw not less than 8 mL / s and can induce a secondary swirl through the curvature of the spiral blades.

3. The integrated cooling device for rotary screw pumping and turbulence dissipation used in spaceborne high-power equipment according to claim 2, characterized in that: v = 6000 ± 500 rpm, α = 45 ± 5°.

4. The integrated cooling device for rotary screw pumping and turbulence dissipation used in high-power spaceborne equipment according to claim 1, characterized in that: R = 0.4 - 0.7r; where r is the radius of the central rod.

5. The integrated cooling device for rotary screw pumping and turbulence dissipation used in spaceborne high-power equipment according to claim 1, characterized in that: With α and R remaining constant, by increasing v and decreasing δ, the heat flux density for heat dissipation of high-power spaceborne equipment can be maintained at no less than 50 W / cm². 2 The mass-to-power consumption ratio does not exceed 1.0W / g.

6. The integrated cooling device for rotary screw pumping and turbulence dissipation used in spaceborne high-power equipment according to claim 5, characterized in that: δ = 0.2 - 0.3r; where r is the radius of the central rod.

7. The integrated cooling device for rotary screw pumping and turbulence dissipation used in spaceborne high-power equipment according to claim 1, characterized in that: Let the thickness of the spiral blade be t, then t = 0.3 ± 0.05 mm.

8. The integrated cooling device for rotary screw pumping and turbulence dissipation used in high-power spaceborne equipment according to claim 1, characterized in that: The cross-section of the cooling pipe is in the shape of a "return", and the roughness Ra of the inner wall surface of the cooling pipe ≤ 0.8 μm.

9. The integrated cooling device for rotary screw pumping and turbulence dissipation used in spaceborne high-power equipment according to claim 1, characterized in that: The material of the swirling disturbance screw is SiC-TiC ceramic matrix composite material, and the material of the cooling pipe is high-purity AlN ceramic.

10. The integrated cooling device for rotary screw pumping and turbulence dissipation for high-power spaceborne equipment according to claim 7, characterized in that: By coordinating the control of α, R, δ, v, and t, the heat flux density during heat dissipation of high-power-density spaceborne chips can be maintained at no less than 50 W / cm². 2 The mass-to-power ratio does not exceed 0.1W / g.

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